Document types are what allow Element 451 to identify which types of documents are needed throughout your instance. Without a Document Type, documents won't be able to be viewed on the user profile.
They are a critical part of Decision451. Any document that you wish to display on an applicants document page there will need to have a document type.
To create a new Document Type, click the plus sign in the top right corner. Give it a name, the slug will populate automatically, and then select an icon for that document.
You can now add that document to a new upload field type under custom fields in your Field Management section.
Once your field is set, you can add it anywhere in Element you would like someone to upload that type of document.